This invention relates to a process for producing xcex1-alumina. xcex1-Alumina powder has been widely used as an abrasive, a raw material for sintered products, a plasma spraying material, a filler and the like. The xcex1-alumina produced by the process of the present invention comprises xcex1-alumina single crystal particles which are not agglomerated particles and have a narrow particle size distribution, and is suitable as an abrasive, a raw material for sintered products, a plasma spraying material, a filler, a raw material for single crystals, a raw material for a carrier for catalysts, a raw material for fluorescent substances, a raw material for encapsulations, a raw material for ceramic filters, etc.
xcex1-Alumina powder obtained by conventional processes comprises irregular-shaped polycrystals, contains many agglomerated particles, and has a broad particle size distribution. For some uses, the purity of conventional xcex1-alumina powder is insufficient. In order to overcome these problems, xcex1-alumina powder produced by special processes as hereinafter described has been employed for specific uses. However, these special processes still fail to arbitrarily control the shape or particle diameter of xcex1-alumina. It has thus been difficult to obtain xcex1-alumina powder having a narrow particle size distribution.
Known special processes for producing xcex1-alumina powder include a process utilizing a hydrothermal reaction of aluminum hydroxide (hereinafter referred to as hydrothermal treatment process); a process comprising adding a flux to aluminum oxide, fusing, and precipitating (hereinafter referred to as flux process); and a process in which aluminum hydroxide is calcined in the presence of a mineralizer.
With respect to a hydrothermal treatment process, JP-B-57-22886 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined published Japanese patent applicationxe2x80x9d) discloses a process in which corundum is added as a seed crystal to control the particle size. The process consists of synthesis in a high temperature under a high pressure, making the resulting xcex1-alumina powder expensive.
According to the study by Matsui, et al. (Hydrothermal Hannou (Hydrothermal Reactions), Vol. 2, pp. 71-78 xe2x80x9cGrowth of Alumina Single Crystal by Hydrothermal Methodxe2x80x9d), an xcex1-alumina single crystal obtained by hydrothermal growth of an alumina single crystal containing chromium on a sapphire (xcex1-alumina) seed crystal contains cracks. On examining the homogeneity of the crystal in an attempt to clarify the cause of the cracks, it was confirmed that a high strain exists in the boundary between the seed crystal and the grown crystal and that the density of etch pit in the grown crystal near the boundary, which seems to correspond to a dislocation density, is high. The report goes that the cracks are expected to relate to such a strain or a defect and that a hydrothermal growth process is easily accompanied by incorporation of a hydroxyl group or water into crystals, which appears to cause a strain or a defect.
A flux process has been proposed as a means for controlling the shape or particle size of xcex1-alumina powder for use as an abrasive, a filler, etc. For example, JP-A-3-131517 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a process comprising calcining aluminum hydroxide in the presence of a flux with fluorine having a melting point of not more than 800xc2x0 C. to prepare xcex1-alumina particles having an average particle diameter of from 2 to 20 xcexcm and a hexagonal plate shape with a D/H ratio of from 5 to 40, wherein D represents a maximum particle diameter parallel to a hexagonal lattice plane of a hexagonal close-packed lattice of xcex1-alumina, and H represents a diameter perpendicular to the hexagonal lattice plane. However, this process cannot provide fine xcex1-alumina powder having a particle diameter of less than 2 xcexcm, and all the particles obtained have a plate shape. It was impossible in the process to arbitrarily control the shape or size of xcex1-alumina particles. Besides, the resulting xcex1-alumina powder is not always suitable for use as an abrasive, a filler and a raw material for single crystals.
The Bayer process is a commonly employed and the least expensive process for producing xcex1-alumina powder. In the Bayer process, bauxite is once converted to aluminum hydroxide or transition alumina, which is then calcined in air to prepare xcex1-alumina powder.
The aluminum hydroxide or transition alumina which is obtained as an intermediate product on an industrial scale at low cost comprises large agglomerates having a particle diameter of greater than 10 xcexcm. Conventional xcex1-alumina powder obtained by calcination of such aluminum hydroxide or transition alumina in air comprises particles of irregular shape containing agglomerated coarse particles. The xcex1-alumina powder containing agglomerated coarse particles are ground into final products by means of a ball mill, a vibration mill, etc., but grinding is not always easy and incurs the cost. xcex1-Alumina powder having difficulty in grinding needs an extended period of time for grinding, during which fine powder may be formed or foreign materials may be incorporated only to provide xcex1-alumina powder unsuitable as an abrasive.
Several proposals have been made to date as a solution to these problems. For example, JP-A-59-97528 discloses a process for improving the shape of xcex1-alumina powder comprising calcining aluminum hydroxide prepared by the Bayer process in the presence of boron containing ammonium and a boron mineralizer to obtain xcex1-alumina powder having an average particle diameter of from 1 to 10 xcexcm and a D/H ratio approximate to 1. However, this process involves problems in that the boron-containing or fluorine-containing material added as a mineralizer remains in the resulting xcex1-alumina and agglomerates are formed upon calcining.
In connection to calcination of sodium-containing aluminum hydroxide prepared by the Bayer process, it has been proposed to conduct calcining in the presence of a fluoride, e.g., aluminum fluoride or cryolite, and a chlorine-containing material, e.g., chlorine or hydrogen chloride in British Patent 990,801 or in the presence of boric acid, and ammonium chloride, hydrochloric acid or aluminum chloride in West German Patent 1,767,511 for the purpose of effectively removing sodium while controlling the particle diameter.
However, in the former process, since a mineralizer such as aluminum fluoride is added in a solid form or the calcination is conducted while supplying chlorine gas and fluorine gas without the addition of water, the resulting alumina particles have problems of an irregular shape and a broad particle size distribution. The latter process also involves a problem in that boric acid as a mineralizer remains in the resulting xcex1-alumina in the form of a boron-containing material. In addition, these processes aim chiefly at removal of sodium, and the sodium salt, such as NaCl or Na2SO4, by-produced by the reaction between sodium and a sodium removing agent must be sublimed or decomposed by calcination at a high temperature of at least 1200xc2x0 C.
With respect to the reaction between alumina and hydrogen chloride gas, there is a report in Zeit. fur Anorg. und Allq. Chem., Vol 21, p. 209 (1932) of an equilibrium constant of the reaction system comprising sintered xcex1-alumina having a particle diameter of from 2 to 3 mm, hydrogen chloride, and produced aluminum chloride. According to the report, while xcex1-alumina is found produced in a place different from the place where the starting material has been charged, only hexagonal plate-shaped particles are obtained.
JP-B-43-8929 discloses a process comprising calcining alumina hydrate in the presence of ammonium chloride to produce alumina having a low impurity content and an average particle diameter of not more than 10 xcexcm. The resulting alumina powder has a broad particle size distribution.
Therefore, none of the conventional techniques succeeded in providing xcex1-alumina comprising xcex1-alumina single crystal particles and satisfying requirements of purity and structural homogeneity of individual particles.
An object of the present invention is to solve the above problems and to provide a process for producing xcex1-alumina in a powder form which comprises homogeneous and non-agglomerated xcex1-alumina single crystal particles from various raw materials. More specifically, it is to provide a process for producing xcex1-alumina in a powder form comprising xcex1-alumina single crystal particles which have an octa- or higher polyhedral shape, a D/H ratio of from 0.5 to 3.0, a narrow particle size distribution, and a high alumina purity, with the individual particles being uniform in composition and freed of structural strain.
The present invention relates to the following inventions.
(1) a process for producing xcex1-alumina characterized in that transition alumina and/or an alumina raw material capable of being converted to transition alumina on heating is calcined in an atmosphere containing at least 1% by volume of hydrogen chloride at a temperature of not less than 600xc2x0 C.
(2) A process for producing xcex1-alumina characterized in that transition alumina and/or an alumina raw material capable of being converted to transition alumina is calcined, while at least 1% by volume of chlorine and at least 0.1% by volume of steam are introduced to an atmosphere, at a temperature of not less than 600xc2x0 C.
(3) A process for producing xcex1-alumina as described in (1) or (2) above, wherein the calcining temperature is from 600 to 1,400xc2x0 C.
(4) A process for producing xcex1-alumina as described in (1) or (2) above, wherein the calcining temperature is from 800 to 1,200xc2x0 C.
(5) A process for producing xcex1-alumina as described in (1), (2) or (3) above, characterized in that the alumina raw material capable of converted to transition alumina on heating is aluminum hydroxide.
(6) A process for producing xcex1-alumina as described in (1), (2) or (3), characterized in that the alumina raw material capable of being converted to transition alumina on heating is alum or aluminum sulfate.
(7) A process for producing xcex1-alumina comprising xcex1-alumina single crystal particles which are homogeneous and have an octa- or higher polyhedral shape and a D/H ratio of from 0.5 to 3.0, wherein D represents a maximum particle diameter parallel to a hexagonal lattice plane of a hexagonal close-packed lattice of the particles, and H represents a diameter perpendicular to the hexagonal lattice plane, characterized in that transition alumina and/or an alumina raw material capable of being converted to transition alumina on heating is calcined in an atmosphere containing at least 1% by volume of hydrogen chloride gas at a temperature of from 600 to 1,400xc2x0 C.
(8) A process for producing xcex1-alumina comprising xcex1-alumina single crystal particles which are homogeneous and have an octa- or higher polyhedral shape and a D/H ratio of from 0.5 to 3.0, wherein D represents a maximum particle diameter parallel to a hexagonal lattice plane of a hexagonal close-packed lattice of the particles, and H represents a diameter perpendicular to the hexagonal lattice plane, characterized in that transition alumina and/or an alumina raw material capable of being converted to transition alumina on heating is calcined, while at least 1% by volume of chlorine and at least 0.1% by volume of steam are introduced to an atmosphere, at a temperature of from 600 to 1,400xc2x0 C.
(9) A process for producing xcex1-alumina comprising xcex1-alumina single crystal particles as described in (7) or (8) above, characterized in that the resulting xcex1-alumina single crystal particles have a deca- or higher polyhedral shape.
(10) A process for producing xcex1-alumina as described in (7) or (8) above, wherein the xcex1-alumina single crystal particles have an average particle diameter of from 0.1 to 30 xcexcm and such a particle size distribution that the D90/D10 ratio is not more than 10, wherein D10 and D90 represent a cumulative 10% diameter and a cumulative 90% diameter, respectively, of a cumulative distribution depicted from the small diameter side.
(11) A process for producing xcex1-alumina as described in (7) or (8) above, characterized in that the xcex1-alumina has an alumina purity of not less than 99.90% by weight.